Cellular telephones are becoming commonplace in today's world. Cellular telephones typically include a full duplex transceiver that can both transmit and receive signals on the frequencies authorized for cellular telephones. Frequency allocations are a limited resource. Thus, in order to accommodate as many communications as possible, the bandwidth of each communication is limited and each communication must be on one channel of a predefined plurality of channels. There is very little guard space, or free bandwidth, available between adjacent channels, so it is critical that each cellular telephone operate precisely within its assigned channel and not drift astray into the bandwidth of an adjacent channel. The regulatory authority of each country, such as the Federal Communications Commission (FCC) in the United States of America, sets the channel frequencies, the channel spacing, and the frequency tolerance. The frequency tolerance is a measurement of how much the cellular telephone may deviate from its allocated frequency. If the tolerance is too high, then the cellular telephone may interfere with the communications on an adjacent channel. If the tolerance is too low, then the cellular telephone will require a very high precision oscillator and the cost of the cellular telephone will be increased.
FCC regulations for cellular telephones specify that a cellular telephone must maintain a frequency error of less than .+-.2.5 parts per million (ppm). To meet this requirement, some cellular telephones use a temperature compensated crystal oscillator (TCXO) which has a frequency error of less than .+-.2.5 ppm. An alternative to the TCXO is an uncompensated voltage controlled crystal oscillator (VCXO). The output frequency of the VCXO is compared with the frequency of the received signal transmitted by the Mobile Telephone Switching Office (MTSO) of the cellular system. The FCC also specifies the tolerance of the MTSO, a frequency error less than 0.2 parts per million. The cellular telephone thus adjusts its own frequency to match the frequency of the MTSO. This is commonly referred to as Automatic Frequency Control (AFC), a well-known method through which a receiver acquires the frequency stability of another source by comparing the frequency of the received signal from that source with the frequency of its own oscillator and then adjusting, as necessary, its own oscillator. Thus, even if the oscillator in the cellular telephone is not a high precision oscillator, or is subject to drift due to aging, temperature, or battery voltage, the receiver in the cellular telephone employing AFC will track the frequency of the received cellular signal from the MTSO to provide a stable signal with the specified frequency tolerance.
However, AFC should not be performed when the received signal is weak or is subject to strong interference because the receiver may track the noise or interference rather than the proper received signal. A problem encountered is that the power of the received signal decreases as the cellular telephone is moved farther away from the cellular tower transmitting the cellular signal, so the relative amount of noise increases. Consider a first case where the received signal is very strong. The measured frequency will be due primarily to the received signal, the standard deviation of the measured frequency of the received signal will be small, and there will be high confidence that the received signal is valid and may be used for AFC operation. As the received signal strength decreases the relative amount of noise will increase, so the standard deviation will increase. However, if the standard deviation is still small, then there still will be confidence that the received signal is valid and may be used for AFC operation. As the received signal strength decreases even further, the standard deviation continues to increase. However, at this point the center frequency of the IF bandpass filter becomes significant. The noise will be gaussian, but centered at the center frequency of the IF bandpass filter. Thus, if the center frequency of the IF bandpass filter is, for example, above the frequency of the IF signal resulting from the received signal, then the noise will be unevenly distributed and will be mostly above the received signal IF frequency. Thus, the measured median frequency will be above the received signal IF frequency so the AFC will move the oscillator frequency so as to center this measured median frequency in the IF bandpass. This will cause the received signal IF frequency to be even lower in the IF bandpass, so the measured median frequency will be above the received signal IF frequency and the AFC will again move the oscillator frequency so as to center this measured median frequency in the IF bandpass. This process continues until the AFC has shifted the oscillator frequency to the point where the received signal IF frequency is so far from the IF center frequency that it has no effect. In other words, frequency lock with the desired signal has been lost.
To help combat this problem, a DC voltage measurement corresponding to the received signal power, also known as a Receiver Signal Strength Indicator (RSSI), is used to determine whether the received signal power is strong enough, or the signal-to-noise ratio is large enough, to ensure that accurate frequency tracking will occur. Although the RSSI is useful, it also suffers from its own problems.
One problem encountered with the RSSI is that it is not useful in representing signal quality over the entire sensitivity range of a receiver, especially near the minimum discernible threshold of the receiver. Therefore, AFC is typically shut off in weak received signal conditions because there is no valid indication of the signal quality. However, shutting off the AFC limits the frequency stability of the transceiver to the frequency stability of its own oscillator. This may be unnecessary because, even if the signal is weak, it may still be strong enough to provide a reference for AFC operation. Therefore, use of only the RSSI may prematurely disable AFC operation.
Another problem encountered with the RSSI is that the RSSI is simply a measure of received signal power in the IF bandpass of the receiver. The RSSI cannot differentiate between a strong interfering signal and the desired reference signal if they are both in the IF bandpass and cannot provide information about whether the interference is strong enough to compromise accurate frequency tracking. For example, consider where the desired received signal is centered at the center frequency of the IF bandpass, but that there is a strong interfering signal up the upper edge of the IF bandpass, or even outside of the IF bandpass if the interfering signal is strong enough. The measured frequency will be between the frequency of the desired received signal and the frequency of the interfering signal. Thus, the measured median frequency will be above the received signal IF frequency so the AFC will move the oscillator frequency so as to center this measured median frequency in the IF bandpass. This will cause the received signal IF frequency to be even lower in the IF bandpass and the interfering signal will be closer to or more within the IF bandpass, so the effect of the interfering signal will be even stronger, and the AFC will again move the oscillator frequency so as to center this measured median frequency in the IF bandpass. This process continues until the AFC has shifted the oscillator frequency to the point where the interfering signal controls the AFC. In other words, frequency lock with the desired signal has been lost. Therefore, even in the presence of a strong received signal, there is the possibility that a stronger interfering signal can pull the transceiver off the desired frequency in the direction of the frequency of the interfering signal.
For the above reasons, and when the fight frequency tolerance requirements are considered, AFC has not been considered to be a highly reliable or highly desirable method of operation of cellular telephones.
Therefore there is a need in the art for an improved method for determining the integrity of a received signal in a frequency tracking system.
There is also a need for a method for determining the integrity of a received signal at levels below which the RSSI does not operate accurately.
There is a further need for a method for determining the integrity of a received signal when there is the possibility of a strong interfering signal.